The instant invention relates to adhesive tapes and more particularly to a method and apparatus for making co-extruded adhesive tapes.
Adhesive tapes made of protective thermoplastic materials have generally been found to be effective for applications, such as protecting above ground, underground and marine pipes against corrosion. Specifically, it has been found that above ground, underground and marine pipes can generally be effectively protected against corrosion by spirally wrapping the exterior surfaces thereof with adhesive tapes in a manner wherein adjacent spiral wrappings overlap each other slightly. Generally it has been found that tapes comprising durable thermoplastic layers made from materials, such as polyethylene and adhesive layers made from elastomeric based adhesives can be effectively utilized for applications of this type. Further, it has generally been found that in many instances it can be advantageous to apply two layers of tapes of this type to pipes in order to provide increased protection against moisture induced corrosion. Nevertheless, it has been found that in some instances moisture can even penetrate through two layers of protective tapes by penetrating the overlapping seam areas of adjacent spiral wrappings. However, recently it has been found that moisture penetration through the overlapping seam areas of protective tape layers can be further reduced by utilizing first or inner tape layers having adhesives on both surfaces thereof and outer tape layers having adhesives on only the inner surfaces thereof. In this regard, it has been found that by utilizing inner tape layers having adhesives on both surfaces thereof, the inner tape layers can more effectively bond to themselves in the overlapping seam areas of adjacent tape sections and the outer tape layers can more effectively bond to the inner tape layers along the entire extents of pipe sections.
Heretofore most conventional tapes for wrapping pipe sections have been manufactured in calendaring processes wherein adhesive layers made from elastomeric based adhesive materials are pressed onto base layers made from thermoplastic materials. However, while tapes of this type have generally been found to be effective for many applications, it has been found that when they are exposed to severe temperature conditions, such as temperatures below -40.degree. F., they tend to delaminate and therefore lose there overall effectiveness.
While most of the heretofore available tapes for wrapping underground pipe sections have been manufactured in calendaring lamination processes, some of the heretofore available tapes have been manufactured in extrusion processes wherein molten thermoplastic streams are merged with molten elastomeric adhesive streams in extrusion dies to form tapes comprising two layers which are more intimately bonded together. However, it has been found that even tapes of this type can become delaminated when they are exposed to extreme temperature conditions due to the generally incompatible nature of elastomeric based adhesives and thermoplastic materials. Further, while attempts have been made to overcome this problem by merging thermoplastic streams and streams comprising thermoplastic-coupling agent blends before they are passed into extrusion dies and then merging the partially stratified combined streams with elastomeric based adhesive streams in extrusion dies, tapes made by this process have been found to be of inconsistent quality, although they have generally been found to be more effective than other types of tapes from a standpoint of resisting delamination.
The instant invention provides an effective method and apparatus for forming an co-extruded adhesive tape which is of consistent quality and highly resistant to delamination under even the most extreme temperature conditions. Specifically, the apparatus of the instant invention comprises a co-extrusion die having an elongated outlet aperture therein and first, second and third extrusion channels which extend from first, second and third inlet openings toward the outlet aperture and merge in the interior of the die to form an outlet channel which opens outwardly through the outlet aperture. The apparatus further includes means for supplying a first pressurized stream comprising an elastomeric base adhesive polymer to the first inlet opening and means for supplying a second pressurized stream comprising a uniformly dispersed blend of a molten flexible thermoplastic and a molten elastomerically compatible coupling agent to the second inlet opening. The apparatus still further includes means for supplying a third pressurized stream comprising either a molten elastomeric based polymer or a molten flexible thermoplastic to the third inlet opening and the preferred embodiment of the apparatus still further includes cooling roller means for cooling the extruded stream after it has passed outwardly from the outlet aperture. The outlet aperture is of elongated dimension in an X direction and reduced dimension in a Y direction which is perpendicular to the X direction. The outlet aperture preferably has a dimension of at least approximately 1 inch in the X direction and a dimension of between approximately 15 mils and 60 mils in the Y direction. Further, the first, second and third channels in the co-extrusion die are preferably of elongated dimension in the X direction and of reduced dimension in the Y direction in the area where the extrusion channels merge. Specifically, the first, second and third channels are preferably of substantially the same dimension in the X direction as the outlet aperture in the area where they merge in the interior of the die.
It has been found that the above described apparatus can be effectively utilized in accordance with the method of the subject invention for forming a continuous sheet of consistently high quality adhesive tape which is highly resistant to delamination. Specifically, the method of the subject invention comprises the steps of continuously and simultaneously passing first, second and third uniformly dispersed molten streams through a co-extrusion die of the above described type. The first stream comprises between approximately 20% and 60% by weight of an elastomeric based adhesive polymer, between approximately 0% and 55% (preferably at least 20%) by weight of fillers, between approximately 8% and 40% by weight of tackifiers, between approximately 0% and 6% (preferably at least 3%) by weight processing aids, between approximately 0% and 60% (preferably at least 15%) by weight of reinforcing materials and between approximately 0% and 15% (preferably at least 2%) by weight of protective materials. The second pressurized stream comprises between approximately 25% and 90% by weight of a flexible thermoplastic and between approximately 5% and 80% by weight of elastomerically compatible coupling agent. The third pressurized stream comprises either between approximately 80% and 100% by weight of a molten flexible thermoplastic or a mixture comprising between approximately 20% and 60% of an elastomeric based adhesive polymer, between approximately 0% and 55% (preferably at least 20%) by weight of fillers, between approximately 8% and 40% by weight of tackifiers, between approximately 0% and 6% (preferably at least 3%) by weight of processing aids, between approximately 0% and 60% (preferably at least 15%) by weight of reinforcing materials, between approximately 0% and 15% (preferably at least 2%) by weight of protective materials. In accordance with the method the first, second and third streams are fed to a co-extrusion die of the above described type so that they pass through the first, second and third extrusion channels respectively, and merge in the outlet channel before passing outwardly through the outlet aperture. The first, second and third channels are oriented so that the material from the second stream is interposed between the material from the first and third streams as the first, second and third streams merge. The first, second and third streams are merged in the die so that they pass outwardly through the outlet aperture as a continuous sheet comprising first, second and third intimately fused layers of material from the first, second and third streams, respectively. The thermoplastic material in the second stream preferably comprises polyethylene and when the third stream includes a thermoplastic material the thermoplastic material in the third stream preferably also comprises polyethylene. The elastomeric based adhesive polymer preferably comprises an adhesive polymer selected from a group consisting of EPDM, EPM or butyl elastomeric based adhesive polymers and the coupling agent in the second stream preferably comprises a coupling agent selected from a group consisting of EPDM, EPM or butyl coupling agents. Further the first, second and third streams are preferably heated to a temperature of at least approximately 300.degree. F. before they merge in the co-extrusion die.
It has been found that the method and apparatus of the subject invention can be effectively utilized for producing high quality tapes having adhesive layers on either one or both sides thereof. In this regard, when the third pressurized stream comprises a molten flexible thermoplastic material the tape is formed so that it has adhesive on only one side thereof, (the side which is defined by material from the first stream). In this case, the second stream provides an intermediate bonding layer which effectively bonds the material from the first stream to the material from the third stream as the three streams are coextruded. The second stream preferably comprises a blend of material from the first and third streams and it is blended so that it is substantially more compatible with both the first and third streams than the first and third streams are with each other. Accordingly, the second stream is able to effectively bond to both the first and third streams. On the other hand, when the third stream comprises an elastomeric based adhesive polymer, the second stream not only functions as a bonding layer between the material from the first and third streams but it also functions as a base or strength layer which provides the necessary level of strength and durability in the tape. In this regard, because the second stream comprises a blend of a flexible thermoplastic and a coupling agent it is able to effectively bond to the first and third elastomeric streams while nevertheless providing the desired level of strength in tapes made in accordance with the method of the instant invention.
Accordingly, it is a primary object of the instant invention to provide an effective method of making a co-extruded adhesive tape which is of consistently high quality and highly resistent to delamination.
Another object of the instant invention is to provide an effective method of forming an adhesive tape by merging lo material from first, second and third molten streams in a three channel extrusion die.
Another object of the instant invention is to provide an effective apparatus for forming an adhesive tape comprising a three channel co-extrusion die.
An even still further object of the instant invention is to provide an effective apparatus for forming a co-extruded adhesive tape which is highly resistent to delamination.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.